Cameras that operate at low light levels have a number of significant applications in diverse areas. These include, among others, photographic, night vision, surveillance, and scientific uses. Modern night vision systems, for example, are rapidly transforming presently used direct view systems to camera based arrangements. These are driven by the continued advances in video display and processing. Video based systems allow remote display and viewing, recording, and image processing including fusion with other imagery such as from a forward looking infra-red sensor. Surveillance applications are also becoming predominately video based where camera size, performance, and low light level sensitivity are often critical. Scientific applications require cameras with good photon sensitivity over a large spectral range and high frame rates. These applications, and others, are driving the need for improved low light level sensors with direct video output.
Image sensing devices which incorporate an array of image sensing pixels are commonly used in electronic cameras. Each pixel produces an output signal in response to incident light. The signals are read out, typically one row at a time, to form an image. Cameras in the art have utilized Charge Coupled Devices (CCD) as the image sensor. Image sensors which incorporate an amplifier into each pixel for increased sensitivity are known as active pixel sensors (sometimes referred to herein as APS). Active pixel sensors are disclosed, for example in U.S. Pat. No. 5,789,774 issued Aug. 4, 1998 to Merrill; U.S. Pat. No. 5,631,704 issued May 20, 1997 to Dickinson et al; U.S. Pat. No. 5,521,639 issued May 28, 1996 to Tomura et al; U.S. Pat. No. 5,721,425 issued Feb. 24, 1998 to Merrill; U.S. Pat. No. 5,625,210 issued Apr. 29, 1997 to Lee et al; U.S. Pat. No. 5,614,744 issued Mar. 25, 1997 to Merrill; and U.S. Pat. No. 5,739,562 issued Apr. 14, 1998 to Ackland et al. Extensive background on active pixel sensor devices is contained in the paper by Fossum, "CMOS Image Sensors: Electronic Camera-On-A-Chip", IEEE Transactions on Electron Devices, Vol. 44, No. 10, pp. 1689-1698, (1997) and the references therein.
In general, it is desirable to provide cameras which generate high quality images over a wide range of light levels including to extremely low light levels such as those encountered under starlight and lower illumination levels. In addition, the camera should have a small physical size and low electrical power requirements, thereby making portable, head-mounted, and other battery-operated applications practical. Active pixel sensor cameras meet the small size and low power requirements, but have poor low light level sensitivity with performance limited to conditions with 0.1 lux (twilight) or higher light levels.
Night vision cameras which operate under extremely low light levels are known in the art. The standard low light level cameras in use today are based on a Generation-III (GaAs photocathode) or Generation-II (multi-alkali photocathode) image intensifier fiber optically coupled to a CCD to form an Image Intensified CCD or ICCD camera. The scene to be imaged is focused by the input lens onto the photocathode faceplate assembly. The impinging light energy liberates photoelectrons from the photocathode to form an electron image. The electron image is proximity focused onto the input of the microchannel plate (MCP) electron multiplier, which intensifies the electron image by secondary multiplication while maintaining the geometric integrity of the image. The intensified electron image is proximity focused onto a phosphor screen, which converts the electron image back to a visible image, which typically is viewed through a fiber optic output window. A fiber optic taper or transfer lens then transfers this amplified visual image to a standard CCD sensor, which converts the light image into electrons which form a video signal. In these existing prior art ICCD cameras, there are five interfaces at which the image is sampled, and each interface degrades the resolution and adds noise to the signal of the ICCD camera. This image degradation which has heretofore not been avoidable, is a significant disadvantage in systems requiring high quality output. The ICCD sensor tends also to be large and heavy due to the fused fiber optic components. A surveillance system having a Generation-II MCP image intensifier tube is described, for example, in U.S. Pat. No. 5,373,320 issued Dec. 13, 1994 to Johnson et al. A camera attachment described in this patent converts a standard daylight video camera into a day/night video camera.
In addition to image degradation resulting from multiple optical interfaces in the ICCD camera a further disadvantage is that the MCP is a relatively noisy amplifier. This added noise in the gain process further degrades the low light level image quality. The noise characteristics of the MCP can be characterized by the excess noise factor, Kf. Kf is defined as the ratio of the Signal-to-Noise power ratio at the input of the MCP divided by the Signal-to-Noise power ratio at the output of the MCP after amplification. Thus Kf is a measure of the degradation of the image Signal-to-Noise ratio due to the MCP gain process. Typical values for Kf are 4.0 for a Generation-III image intensifier. A low noise, high gain. MCP for use in Generation-III image intensifiers is disclosed in U.S. Pat. No. 5,268,612 issued Dec. 7, 1993 to Aebi et al.
An alternate gain mechanism is achieved by the electron-bombarded semiconductor (sometimes referred to herein as EBS) gain process. In this gain process, gain is achieved by electron multiplication resulting when the high velocity electron beam dissipates its energy in a semiconductor. The dissipated energy creates electron-hole pairs. For the semiconductor silicon one electron-hole pair is created for approximately every 3.6 electron-volt (eV) of incident energy. This is a very low noise gain process with Kf values close to 1. A Kf value of 1 would indicate a gain process with no added noise.
The electron-bombarded semiconductor gain process has been utilized in a focused electron bombarded hybrid photomultiplier tube comprising a photocathode, focusing electrodes and a collection anode consisting of a semiconductor diode disposed in a detector body as disclosed in U.S. Pat. No. 5,374,826 issued Dec. 20, 1994 to LaRue et al. and U.S. Pat. No. 5,475,227 issued Dec. 12, 1995 to LaRue. The disclosed hybrid photomultiplier tubes are highly sensitive but do not sense images.
The electron-bombarded semiconductor gain process has been used to address image degradation in the ICCD low light level camera. A back illuminated CCD is used as an anode in proximity focus with the photocathode to form an Electron Bombarded CCD (EBCCD). Photoelectrons from the photocathode are accelerated to and imaged in the back illuminated CCD directly. Gain is achieved by the low noise electron-bombarded semiconductor gain process. The EBCCD eliminates the MCP, phosphor screen, and fiber optics, and as a result both improved image quality and increased sensitivity can be obtained in a smaller sized camera. Significant improvement of the degraded resolution and high noise of the conventional image transfer chain has been realized with the EBCCD. An EBCCD is disclosed in U.S. Pat. No. 4,687,922 issued Aug. 18, 1987 to Lemonier. Extensive background on EBCCDs is contained in the paper by Aebi, et al, "Gallium Arsenide Electron Bombarded CCD Technology", SPIE Vol. 3434, pp. 37-44, (1998) and references cited therein.
Optimum low light level EBCCD performance requires a specialized CCD. The CCD is required to be backside thinned to allow high electron-bombarded semiconductor gain. The CCD cannot be used in a frontside bombarded mode as used in a standard CCD camera as the gate structures would block the photoelectrons from reaching the semiconductor and low electron-bombarded semiconductor gains would be obtained at moderate acceleration voltages High acceleration voltages required to penetrate the gate structures would cause radiation damage to the CCD and shorten CCD operating life. Also a frame transfer format is required where the CCD has both an imaging region and a store region on the chip. The image and store regions are of approximately the same size. A frame transfer format is required for two reasons. First it is essential that the CCD imaging area have high fill factor (minimum dead area) if possible. The frame transfer CCD architecture satisfies this requirement. The interline transfer CCD architecture would result in substantial dead area (of order 70-80%). Any reduction in active area will result in lost photoelectrons. This is equivalent to a reduction in photocathode quantum efficiency or sensitivity. At the lowest light levels (starlight or overcast starlight), low light level camera performance is dictated by the photon statistics. It is essential that the maximum number of photons be detected by the imager for adequate low light level resolution and performance. Second a frame transfer format allows signal integration to occur during the readout of the store region in addition to any integration period. This allows charge to be integrated almost continuously maximizing the collected signal.
EBCCD cameras have several other disadvantages. The frame transfer CCD architecture has the serious disadvantage for the EBCCD application of essentially doubling the size of the required vacuum envelope due to the requirement for image and store regions on the CCD. This requirement also means that the frame transfer CCD chip is over twice the size of the image area. This substantially increases the cost of the COD relative to interline transfer CCDs or active pixel sensor chips as fewer chips can be fabricated per silicon wafer. EBCCD based cameras also have the disadvantage of backside illumination of the CCD which necessitates specialized processing to thin the semiconductor and passivate the back surface for high electron-bombarded semiconductor gain. This processing is not standard in the silicon industry and substantially increases the EBCCD manufacturing cost. The EBCCD cameras consume several watts of power due to the CCD clocking requirements and require external electronics for a complete camera. The size of the external camera electronics presents an obstacle to applications that would benefit from miniaturization of the camera. Finally CCDs require specialized semiconductor processing lines which are not compatible with mainstream CMOS semiconductor fabrication technology. This further increases the cost of CCD based cameras.